Oxidation reactors and processes can be classified into two types. One type of apparatus comprises a combustor or burner for burning a fuel with an oxidant, such as air, into complete, that is deep, oxidation products, specifically, carbon dioxide and water, with production of heat of reaction. Burners find utility, for example, in cooking appliances and in providing heat to a heater head of an external combustion engine, such as, a Stirling engine. Complete combustion processes take place under fuel-lean reaction conditions in an excess of oxygen, as shown in Equation (1):CnHm+(n+¼m)O2→½mH2O+nCO2  Equation (1)Another type of oxidation apparatus comprises a fuel reformer for converting a hydrocarbon fuel with an oxidant into one or more incomplete, that is partially-oxidized, reaction products and heat of reaction. Preferably, the partially-oxidized reaction product comprises a gaseous fuel, preferably, synthesis gas comprising a mixture of carbon monoxide and hydrogen. Synthesis gas finds use in Fisher-Tropsch processes for preparing alcohols. Hydrogen obtainable from synthesis gas is a high value clean-burning fuel with applications in fuel cells and hydrogenation reactors. Partial or incomplete oxidation processes occur under fuel-rich conditions, wherein the quantity of oxidant is restricted relative to the quantity of fuel as shown in Equation (2):CnHm+½nO2→½mH2+nCO  Equation (2)Steam may be co-fed to the partial oxidation process to facilitate autothermal reforming.
Both deep and partial oxidation processes can be conducted in the presence or absence of a catalyst. In the absence of a catalyst, deep or partial oxidation occurs thermally; often such oxidation processes produce a flame. Use of a catalyst can reduce operating temperature and improve efficiency and selectivity to desired oxidation products. Moreover, catalytic oxidation is flameless, which provides for a lower noise profile.
The art discloses, for example in US 2008/0078175-A1, a catalytic combustor that provides heat to a heater head of an external combustion engine. The combustor comprises a high-pressure electromagnetic fuel injector for atomizing a liquid fuel into a combustion chamber wherein the fuel is vaporized by heating with a glow plug and/or by contact with a preheated flow of oxidant. The combined mixture of vaporized fuel and oxidant is contacted with a combustion catalyst, which is typically lit-off by means of a glow plug or spark plug, thereby producing deep oxidation products with exothermic heat of reaction. The heat of reaction thusly produced is transferred, in part, to the heater head of a Stirling engine or another heat acceptor surface and, in part, recuperated for heating the flow of oxidant.
The art also discloses, for example in US 2007-0151154-A1, a catalytic partial oxidation reformer, wherein a liquid fuel is delivered through a high-pressure electromagnetic fuel injector into a mixing chamber, wherein the fuel is heated and vaporized by means of a glow plug and/or contact with a preheated flow of oxidant and optionally steam. The combined mixture of vaporized fuel, oxidant, and optional steam is converted on contact with a reforming catalyst into one or more partially-oxidized reaction products, namely, carbon monoxide and hydrogen, with exothermic heat of reaction.
The art would benefit from development of burners and reformers that are light-weight, compact, and portable, with a low noise profile. A burner or reformer of this size and portability typically employs a fuel input greater than about 50 watts thermal (50 Wth) and less than about 5 kilowatts thermal (5 kWth). The burner or reformer is desirably operated with a logistics fuel, such as diesel or jet propulsion fuel, preferably, over a time frame on the order of at least about 100 hours, and preferably, about 1,000 hours. Logistics fuels are distillate fuels. For the purposes of this invention, the term “distillate fuel” refers to a fuel oil obtained as a distillate fraction in refinery operations. A burner or reformer with the aforementioned features would meet requirements and possess desirable advantages for use in logistics, that is, field operations.
In large scale combustion and reforming reactors lacking portability, the high-pressure electromagnetic fuel injector provides an acceptable method for vaporizing a liquid fuel. Typically, such fuel injectors comprise a nozzle rated for a minimum fuel flow of about 19 cubic centimeters per minute (19 cm3/min) at a pressure of about 100 pounds per square inch (100 psi; 689 kPa). These nozzles might be capable, at best, of a 2:1 downturn to a fuel flow of about 9.5 cm3/min at 100 psi (689 kPa). In contrast, a compact and portable combustor or reformer employing a fuel input of about 300-400 watts thermal (300-400 Wth), optionally combined with a generating system having an output of about 80 watts electric (80 We), requires a fuel flow between about 0.5 cm3/min at 100 psi (689 kPa) and about 1 cm3/min at pressures considerably less than 100 psi (689 kPa). These values are well below the usable minimum flow rate and pressure rating of conventional high-pressure electromagnetic injector nozzles. Moreover, at such low flow rates, the nozzle tends at high operating temperature to coke due to clogging of its flow path and/or orifice from fuel breakdown or coking. High-pressure electromagnetic injectors are incapable of operating with heavy fuels in combustors and reformers of compact and portable size for a time frame of 100 hours; let alone for the more than 1,000 hours that might be preferred for logistics operations. Moreover, high pressure electromagnetic injectors typically require air compression, which increases weight and parasitic power requirements and decreases efficiency and portability in a stand-alone system.
Other means of vaporizing a liquid fuel are known, but they too are not suitable for use with distillate fuels in logistics operations. Conventional electrostatic atomization for vaporizing a liquid fuel has the advantages of large fuel passages and acceptable function, but disadvantageously requires high voltage. Also, electrostatic atomization disadvantageously requires conductivity-enhancing chemical additives in the fuel and runs the risk of fuel ignition from electrical charge build-up during transient operation. Ultrasonic vaporization and piezoelectric vaporization are less conventional methods that are disadvantageously costly and undependable.
In view of the above, a need exists in the art to provide an apparatus and a method for vaporizing and, optionally, igniting a liquid fuel, preferably a distillate fuel, in a manner adaptable for use in compact, low weight, and portable combustors and fuel reformers. It would be more advantageous if such an apparatus were to have a low noise profile and could operate efficiently over a time frame of at least about 100 hours and, preferably, longer. It would be even more advantageous if such an apparatus had a low parasitic requirement, that is, a low energy demand from subsidiary pumps and blowers. All of the aforementioned attributes would render the vaporization apparatus and method desirable for use in logistics and field operations.